1. Field of the Invention
The present invention relates to a rotary head-type recording and/or reproducing apparatus, and particularly to a rotary head-type recording and/or reproducing apparatus suitable for, for example, recording and/or reproducing digital signals.
2. Description of the Related Art
As is generally known, an apparatus for recording and/or reproducing large amounts of digital signals such as video signals at a high speed must be equipped with many heads which simultaneously contact a tape for decreasing a bit rate per signal system by distributing signals to the many heads. FIG. 1 shows an example of the arrangement of rotary heads in a digital recording and/or reproducing apparatus equipped with eight heads on which a tape is wound for 180.degree.+.alpha. for distributing signals to four systems.
In FIG. 1, heads 2A to 2H provided on the periphery of a rotating drum 1 at equal intervals and the same height successively contact and separate from a magnetic tape 3 with the rotation of the rotating drum 1. Since at least four heads simultaneously contact the tape 3, four signals can be recorded or reproduced. The track pattern shown in FIG. 2 is thus formed on the magnetic tape 3 at a track pitch T substantially proportional to the feed speed of the tape 3. In this example, the azimuth angles of adjacent heads are respectively set to +A.degree. and -A.degree. so that a high recording density can be obtained by a so-called azimuth overwrite technique, without using a guard band.
In the arrangement of the rotary heads described above with reference to FIGS. 1 and 2, there is a demand for changing a bit rate of digital signals which can be recorded and/or reproduced. Namely, there is a demand for recording and/or reproducing signals for a longer time by decreasing the information amount using the technique of band compression or the like, though this tends to deteriorate the image quality to some extent.
In the above example, the demand can be satisfied by the following three possible methods:
(1) A method of changing the tape speed alone. PA1 (2) A method of proportionally changing the tape speed and the rotational speed of the rotating drum. PA1 (3) A method of proportionally changing the tape speed and the number of the heads used. PA1 (1) A lowest bit rate mode in which the first and second adjacent heads having different azimuth angles are alternately used for each rotation. PA1 (2) A double bit rate mode in which the third head arranged at a position offset, by a predetermined phase, from the position located 180.degree. from one of the first and second heads, and the other head of the first and second heads are used. PA1 (3) A quadruple bit rate mode in which one of the first and second heads, the fourth and fifth heads arranged at positions offset, by a predetermined phase, from a position located 90.degree. from the one of the first and second heads, and the third head are used.
However, all of these methods have critical problems. The problems with these methods are described below with reference to a case where the bit rate is halved, and the recording time is doubled.
In the first method, the track pitch is halved, as shown in FIG. 3, thereby decreasing the S/N (signal-to-noise) ratio by about 3 dB. Although an analog recording type apparatus generally employs this method, this method increases the error rate of data in recording of digital signals and thus has significant adverse effects on recording.
In the second method, although the track pitch is constant, the rotational speed of the rotating drum 1 is halved, thereby causing mechanical troubles such as an increase in rotational unevenness, poor contact between a head and a tape, and the like. There is also an electrical problem that since the frequency of the signal handled by the heads is halved, a transmission system comprising a reproduction amplifier, an equalizer or the like cannot be optimized.
In the third method, although two-system parallel recording is performed by using the heads 2A, 2C, 2E and 2G (refer to FIG. 4), crosstalk occurs due to the use of the four heads having the same azimuth angle. The use of the other heads causes trouble in that the track pitch varies.
In consideration of the above points, the inventors have previously proposed a rotary head-type recording and/or reproducing apparatus which is arranged so that recording and/or reproducing can be realized with a constant track pitch in spite of a change in the tape speed.
This apparatus is described below.
FIG. 5 is a drawing illustrating the arrangement of rotary heads in a digital video tape recorder (DVTR) as an example of the above apparatus. In FIG. 5, reference numerals 2A, 2C, 2E and 2G each denote a head with a first azimuth angle, and reference numerals 2B, 2D, 2F and 2H each denote a head with a second azimuth angle. The heads 2A, 2C, 2E and 2G and the heads 2B, 2D, 2F and 2H are alternately arranged on the periphery of a rotating drum 1 at equal intervals. The heights of these heads in the direction of the rotational axis are adjusted to be the same. Reference numerals 2C' and 2G' denote heads with the second azimuth angle which are provided adjacent to the heads 2C and 2G, respectively, with the first azimuth angle, and being at a height having a predetermined difference X (not shown) in height therefrom. The heads 2C' and 2G' are arranged so as to have substantially the same trace locus as that of the heads 2C and 2G when a tape is moved at a predetermined speed of 1/2 V.sub.T, as shown in FIG. 6, which shows the case of the head 2C'. Reference numeral 3 denotes the magnetic tape which is wound around the rotating drum 1 for 180.degree.+.alpha..
In this arrangement, when the magnetic tape 3 is moved at a speed of V.sub.T while the rotating drum 1 is rotated at a predetermined rotational speed using the eight heads 2A, 2B, 2C, 2D, 2E, 2F, 2G and 2H, and when four of the eight heads, which simultaneously contact the tape, are successively selected for recording four signals, the track pattern shown in FIG. 2 is obtained, as in the above-described conventional example.
When the tape speed is then set to 1/2 V.sub.T, and when two of the four heads 2A, 2C', 2E and 2G', which simultaneously contact the tape, are successively selected for recording two signals, the track pattern shown in FIG. 6 is obtained. Namely, although the track angle is only slightly changed, the track pitch T is the same as that at the standard speed, and the S/N ratio of the recorded and reproduced signals is constant.
The above arrangement permits recording and reproducing both in a standard speed mode using the heads 2A to 2H and a 1/2 speed mode using the heads 2A, 2C', 2E and 2G'.
However, in the above arrangement, if recording and reproducing are performed at 1/4 speed by further decreasing the bit rate, further consideration must be made for keeping the track pitch constant.
Namely, a track pattern with a substantially constant track pitch can be obtained when the heads 2C and 2G' or 2C' and 2G are used. However, since the difference X in height, which is provided to correspond with the phase interval between the heads 2C and 2C' and the heads 2G and 2G' is suitable for the 1/2 speed mode, the difference in height is too large by 1/2 X for the 1/4 speed mode achieved by decreasing the bit rate, thereby producing a track pitch increased by 1/2 X and a track pitch decreased by 1/2 X.
When the recording density can be increased by improving the characteristics of the tape and heads, the track pitch is decreased by decreasing the tape speed so that the recording time can be increased. However, since an appropriate difference X in height is determined in proportion to the track pitch, nonuniformity significantly occurs in the track pitch in the above example. It is thus necessary to add a head for a long-term mode.